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Thunderstorm

Thunderstorm

A storm is any disturbance in Earth's atmosphere with strong winds accompanied by rain or snow and sometimes thunder and lightning. Storms have a generally positive effect on the environment and on human societies because they are the source of most of the precipitation on which the planet depends.

The most common violent change in the weather is the thunderstorm. In the United States, thunderstorms usually occur in the late spring and summer. Thunderstorms are rare in the parts of the country where the air tends to be colder, such as the New England states, North Dakota, and Montana. They also are rare by the Pacific Ocean, where summers are dry. The southeastern states tend to have the most thunderstorms. Some parts of Florida experience thunderstorms on a average of 100 days a year. A thunderstorm may last up to two hours, but most thunderstorms peak after about 15 to 30 minutes.

How thunderstorms form

Thunderstorms develop by the same process that forms cumulus clouds, the puffy clouds of summer skies. These clouds form when a humid air mass (air with an abundance of water vapor) near the surface rises on currents of air called updrafts. As the air mass rises through the atmosphere it expands and cools. Eventually, the rising air cools to the point where its water vapor condenses to form droplets of liquid water, releasing heat in the process into the surrounding air. This latent heat, in turn, causes the air mass to rise ever more quickly. The upward movement of air in a storm cloud has been measured at more than 50 miles (80 kilometers) per hour.

As the upward movement of air continues, more moisture condenses out of the air mass and the suspended droplets form a large cloud. Depending on atmospheric conditions, a storm cloud of this type may rise to a height of anywhere from 6 to 9 miles (10 to 15 kilometers). In the clouds of colder climates, droplets may freeze to form ice crystals, which grow as more and more water vapor condenses on them. The droplets or ice crystals only grow as long as they can be supported by the updrafts. When they grow too large they begin to fall out of the cloud as drizzle or raindrops.

Words to Know

Latent heat: The heat given off when water vapor condenses to form liquid water.

Riming: The freezing on contact of raindrops as they collect on an ice pellet growing to a hailstone.

Updraft: Any movement of air away from the ground.

If the updrafts in the cloud are vigorous enough, much larger precipitation will be formed. In a severe storm, some of the ice crystals may be dragged down by the downdrafts, then swept up again by updrafts. Ice particles may be circulated several times through the storm cloud in this manner, picking up water with each cycle. In a process called riming, raindrop water freezes onto the ice particles, eventually producing large hailstones. Hailstones continue to be recirculated through the cloud until they grow large enough to fall out under their own weight. If located in the right part of the storm, hailstones can grow to impressive sizes. Hail as large as 5.5 inches (14 centimeters) in diameter has been recorded.

Lightning and thunder

Another product of the vigorous up and down drafts in the storm cloud is lightning. Lightning is a giant spark caused by a buildup of static electrical charges. By processes that still are not fully understood, thunderstorm clouds build up a large concentration of positive electrical charges near the top of the cloud and negative electrical charges near the middle. Usually the cloud base has a smaller pocket of positive charge. These opposite charges result in huge voltage differences within the cloud and between the cloud base and the ground. The opposite charges are strongly attracted to each other and when the air between them can no longer keep them apart, a discharge takes placea bolt of lightning. Depending upon the location of the opposite charges, lightning can occur as cloud-to-ground lightning, cloud-to-cloud lightning, or cloud-to-air lightning.

The temperature of a lightning bolt exceeds 40,000°F (22,000°C). The surrounding air is superheated, causing it to expand and then contract rapidly. This expansion and contraction produces the sound vibrations heard as thunder.

It is possible to calculate how far away a storm is by counting the seconds between a lightning flash and a thunder clap. Since it takes thunder about 5 seconds to travel 1 mile (3 seconds to travel 1 kilometer), simply divide the counted seconds by 5 to determine the miles (by 3 to determine the kilometers). Normally, thunder cannot be heard more than 20 miles (32 kilometers) away.

[See also Air masses and fronts; Cyclone and anticyclone; Tornado ]

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thunderstorm

thunderstorm, violent, local atmospheric disturbance accompanied by lightning, thunder, and heavy rain, often by strong gusts of wind, and sometimes by hail. The typical thunderstorm caused by convection occurs when the sun's warmth has heated a large body of moist air near the ground. This air rises and is cooled by expansion. The cooling condenses the water vapor present in the air, forming a cumulus cloud. If the process continues, the summit often attains a height of 4 mi (6.5 km) above the base, and the top spreads out in the shape of an anvil. The turbulent air currents within the cloud cause a continual breaking up and reuniting of the raindrops, which may form hail, and builds up strong electrical charges that result in lightning. As the storm approaches an area, the gentle flow of warm air feeding the cloud gives way to a strong, chilly gust of wind from the opposite direction, blowing from the base of the cloud. Intense rain begins, then gradually diminishes as the storm passes. Night thunderstorms are caused by the cooling of the upper layers of air by radiation; others are caused by approaching cold air masses that advance as a wedge near the ground, forcing the warmer air in its path to rise. Even a forest fire or a volcanic eruption may create a thunderstorm. Thunderstorms occur most frequently in the equatorial zone (some localities have as many as 200 a year) and seldom in the polar regions. In the United States they are most frequent along the E Gulf Coast (averaging more than 70 a year) and least frequent on the Pacific coast (less than 4 a year).

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thunderstorm

thunderstorm Electrical storm caused by the separation of electrical charges in clouds. Water drops are carried by updraughts to the top of a cloud, where they become ionized and accumulate into positive charges – the base of the cloud being negatively charged. An electrical discharge (a spark) between clouds, or between a cloud and the ground, is accompanied by light (seen as a lightning stroke) and heat. The heat expands the air explosively and causes it to reverberate and produce sounds and echoes called thunder.

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thunderstorm

thunderstorm A storm of fairly local scale in which strongly developed cumulonimbus cloud produces thunder and lightning, usually with rain and strong, gusting wind, and often with hail.

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thunderstorm

thun·der·storm / ˈ[unvoicedth]əndərˌstôrm/ • n. a storm with thunder and lightning and typically also heavy rain or hail.

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thunderstorm

thunderstormconform, corm, dorm, form, forme, haulm, lukewarm, Maugham, misinform, norm, outperform, perform, shawm, storm, swarm, transform, underperform, warm •landform • platform • cubiform •fungiform, spongiform •aliform • bacilliform •cuneiform, uniform •variform • vitriform • cruciform •unciform • retiform • multiform •oviform • triform • microform •chloroform • cairngorm • sandstorm •barnstorm •brainstorm, rainstorm •windstorm • snowstorm • firestorm •thunderstorm

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